warn when passing bins and norm to hexbin

[beckermr]

Description

I don't have a full example worked up, but hexbin plots come out wrong if you pass both the bins and norm keywords. I am used to the bins keyword and so passed it by habit. I didn't realize this was not the right API. I realize proplot != matplotlib, but it might be good to warn if both are passed. matplotlib itself actually does this in some cases, but not all cases. Given that proplot doesn't use bins at all AFAICT, i'd be easiest to remove if it is found and warn the user I think?

Steps to reproduce

A "Minimal, Complete and Verifiable Example" will make it much easier for maintainers to help you.

# your code here
# we should be able to copy-paste this into python and exactly reproduce your bug

Expected behavior: [What you expected to happen]

Actual behavior: [What actually happened]

Equivalent steps in matplotlib

Please try to make sure this bug is related to a proplot-specific feature. If you're not sure, try to replicate it with the native matplotlib API. Matplotlib bugs belong on the matplotlib github page.

# your code here, if applicable
import matplotlib.pyplot as plt

Proplot version

Paste the results of import matplotlib; print(matplotlib.__version__); import proplot; print(proplot.version)here.